Currently we are in the midst of an information revolution typified by the growing popularity of the internet and the worldwide web in particular. Supposedly we are on the brink of an information explosion where all types of data from current news to first run motion pictures on demand will be delivered to our homes. The big question is how this delivery will be accomplished. The bandwidth of telephone lines and even coaxial television cables is not adequate to the task. While it is possible that miniature satellite dishes could provide adequate bandwidth, it seems unlikely that such satellite service could provide the interactive communication envisioned by many experts. Therefore, the most likely means for delivering adequate data bandwidth to every home seems to be fiber optical cable. If fiber optical systems are to be successful, inexpensive and reliable optical transmitters (modulators) and receivers (demodulators) are essential.
The most promising modulators are various electro-optic devices such as a Mach-Zehnder waveguide modulator, in which an input light signal is separated into two light signals travelling two separate optical paths. An electric field modifies the optical path length of one of the paths, thus, phase modulating light passing through that path. At the output end the two signals are recombined and light interference converts the phase modulation into an amplitude modulation. These systems are highly favored because many of the essential components can be produced by integrated fabrication techniques analogous to those used in production of microelectronics. This should allow the production of small, inexpensive, highly reliable "integrated optic" devices.
Unfortunately, the current electro-optic modulating devices are extremely sensitive to the polarization plane of incoming light. This necessitates precise alignment of the laser relative to the modulator. Further, the light source must be extremely stable lest the plane of polarization change with time, thereby degrading the efficiency of the modulator.
Not only is the polarization of lasers apt to drift; optical fibers usually alter the polarization of light beams passing therethrough. This problem is greatly exacerbated if the fibers are bent or otherwise stressed. While there are special optical fibers that have virtually no effect on polarization, these polarization preserving fibers are much more expensive than the ordinary optical fiber. Therefore, there is considerable need for modulators that are immune to changes in polarization.
There has been considerable inventive effort expended in producing optical fiber systems that are insensitive to the polarization of incoming optical signals. One solution to the problem is to produce a receiver (demodulator) that is polarization insensitive. U.S. Pat. No. 4,718,120 to Tzeng uses a pair of polarization beamsplitters and a pair of delay demodulators which are sensitive to the outputs of a pair of optical balanced receivers to eliminate effects due to the plane of polarization of the incoming optical signal. U.S. Pat. No. 5,060,312 to Delavaux discloses a polarization insensitive coherent lightwave detection system using only a single polarization beamsplitter. U.S. Pat. No. 5,140,277 to Hooijmans et al. provides a different design for a polarization independent receiver employing one polarization beamsplitter. U.S. Pat. No. 5,142,402 to Tsushima et al. discloses a polarization diversity optical receiver employing one polarization beamsplitter and a frequency converter. U.S. Pat. No. 5,307,197 to Tanabe et al. discloses a simple polarization insensitive receiver based on three polarization beamsplitters and a double balanced receiver diode arrangement.
Another possible solution is to scramble the polarization of the signal beam by forcing signal polarization alternately into one or the other of two orthogonally polarized signal components. With this approach it is not possible for a polarization sensitive receiver to be simultaneously orthogonal to both signal components. At least one of the signal components will always be received, and if the scrambling frequency is sufficiently high compared to the data bit rate, the original data signal can be recovered. U.S. Pat. No. 5,008,958 to Cimini, Jr. et al. varies the polarization state of either the optical signal or the local optical oscillator (laser) using single mode high birefringent fibers to implement polarization switching. U.S. Pat. No. 5,031,236 to Hodgkinson et al. employs a voltage controlled optical switch to vary signal polarization by switching the optical signal alternately to one of two optical paths.
Yet another possibility is to use polarization modulation or switching in place of phase or amplitude modulation. With polarization switching the two binary data states produce orthogonally polarized signals. This way, at least one of the binary states will always be received by a polarization sensitive receiver, allowing the other state to be reconstructed, if necessary, from a properly coded data stream. Polarization switching can be accomplished by converting a phase modulated signal as is disclosed in U.S. Pat. No. 5,293,264 to van Deventer, where a high-order retarder is used to convert a phase modulated optical signal. Switching can also be directly accomplished by a special modulator, as is disclosed in U.S. Pat. No. 5,069,520 to Calvani et al.
Although the above mentioned devices and methods intends to solve polarization problems in heterodyne receivers in coherent optical communication systems. None of them concerns with polarization problems of modulators in the transmitters, especially when the laser source is located a distance away from the modulator.